Subgraph OS was designed from the ground-up to reduce the risks in endpoint systems so that individuals and organizations around the world can communicate, share, and collaborate without fear of surveillance or interference by sophisticated adversaries through network borne attacks.

Subgraph OS is designed to be difficult to attack. This is accomplished through system hardening and a proactive, ongoing focus on security and attack resistance. Subgraph OS also places emphasis on the integrity of installable software packages.

Apparently they devoted all their effort into creating a new email client:

"Subgraph OS was designed to enable secure communication, and a key part of a secure communications platform is the email client. Subgraph has written an entirely new email client from the ground-up to be a usable, attack resistant, standards-supporting end-user client for communicating securely with email."